The present invention relates to a process for recovering sucrose from low grade sugar syrups, juices, or liquors, such as molasses, that also contain a significant concentration of invert sugars.
The production of cane sugar for human consumption generally comprises two distinct operations, namely the production of raw sugar and the production of refined sugar. Production of raw sugar typically takes place at a sugar mill. In the mill, sugar cane stalks are chopped into pieces and the pieces are crushed in a series of mills in order to remove the juice. The juice from the first set of roller mills is referred to as xe2x80x9cfirst juice,xe2x80x9d while the total juice from all the roller mills in the process is referred to as xe2x80x9cmixed juice.xe2x80x9d The juice is normally limed, deaerated and clarified (i.e., removal of suspended solids, usually by sedimentation). The clarified stream is referred to as xe2x80x9cclarified juice.xe2x80x9d The juice is then evaporated to a thick syrup (known as xe2x80x9cevaporated juicexe2x80x9d or xe2x80x9cthick juicexe2x80x9d), and crystallized in a vacuum pan. The xe2x80x9cmassecuitexe2x80x9d (i.e., mixture of sugar syrup and crystals) produced in the vacuum pan is stirred in a crystallizer, and the mother syrup is spun off from the raw sugar crystals in a centrifugal separator. The solid sugar in the centrifugal basket is washed with water to remove remaining syrup. The solid crystalline product is termed xe2x80x9craw sugar.xe2x80x9d The syrup remaining after multiple stages of crystallization and centrifugation is referred to as xe2x80x9ccane mill molassesxe2x80x9d and is typically used for animal feed or fermentation syrups.
Raw sugar from the mill is usually transported to a sugar refinery for further processing. In a conventional cane sugar refining process, the raw sugar is first washed and centrifuged to remove adherent syrup, and the xe2x80x9caffined sugarxe2x80x9d thus produced is dissolved in water as xe2x80x9cmelter liquor.xe2x80x9d The syrup removed from the surface of the raw sugar is known as xe2x80x9caffination syrupxe2x80x9d and is broadly similar in composition to the mother syrup from the raw sugar crystallization. The affination syrup is processed in a xe2x80x9crecovery sectionxe2x80x9d through a series of vacuum pans, crystallizers and centrifugal separators similar to those used for the production of raw sugar, to recover an impure crystalline sugar product which has approximately the same composition as raw sugar. This recovered sugar product is dissolved in water, along with the affined raw sugar, to make melter liquor. The syrup remaining after the multiple stages of crystallization and centrifugation is referred to as xe2x80x9ccane refinery molasses,xe2x80x9d and is typically used for animal feed or fermentation syrups.
The melter liquor is purified, generally by the successive steps of clarification (also referred to as xe2x80x9cdefecationxe2x80x9d) and decolorization, and the resulting xe2x80x9cfine liquorxe2x80x9d is crystallized to give refined sugar (also known as xe2x80x9cwhite sugarxe2x80x9d). The clarification step usually involves forming an inorganic precipitate in the liquor, and removing the precipitate and along with it insoluble and colloidal impurities which were present in the melter liquor. In one of the clarification processes commonly used for melter liquor, termed xe2x80x9ccarbonatationxe2x80x9d or xe2x80x9ccarbonation,xe2x80x9d the inorganic precipitate is calcium carbonate, normally formed by the addition of lime and carbon dioxide to the liquor. The calcium carbonate precipitate is usually removed from the liquor by filtration. Other clarification processes, termed phosphatation processes, involve adding lime and phosphoric acid to the liquor, and produce calcium phosphate precipitate.
The molasses produced in cane mills and refineries contains a substantial concentration of sucrose (e.g., 35-55% by weight on a dry solids basis). However, that sucrose cannot be recovered readily by additional crystallizations, because the molasses contains such a high concentration of impurities, including invert sugars (a mixture of glucose and fructose). The sucrose in the molasses could be sold for a far higher price than the molasses, if only the sucrose could be separated from the other constituents of the molasses in an economical way. However, the prior art has failed to provide a practical and cost-effective way to make this separation for cane syrups where invert is a significant component.
Chromatographic separation is used to desugar beet molasses and is being proposed for cane, but beet molasses has no invert and it is more straightforward to separate the sucrose. Chromatographic separation is an expensive process for cane. There is a long-standing need for improved processes for enhancing recovery of sucrose from low grade cane syrups such as molasses.
The present invention concerns a process for obtaining sucrose from a feed syrup that comprises sucrose and no less than about 3% by weight invert sugars (on a dry solids basis). The process involves nanofiltration of this feed using a membrane, whereby a nanofiltration permeate and a nanofiltration retentate are produced. The nanofiltration permeate will comprise invert sugars that have passed from the feed through the nanofiltration membrane, and preferably will also comprise ash and organic acids. The nanofiltration retentate has (1) a concentration of sucrose that on a dry solids basis is higher than the concentration of sucrose in the feed syrup, and (2) a concentration of invert sugars that on a dry solids basis is lower than the concentration of invert sugars in the feed syrup. The nanofiltration retentate is recovered and sucrose can be crystallized therefrom. The reduction of the invert content facilitates crystallization and thus enhances sucrose recovery.
Preferably, the feed syrup comprises at least about 5%, more preferably at least about 15% invert sugars on a dry solids basis. Suitable feed syrups include, for example, cane mill molasses, cane refinery molasses, and beet molasses, as well as a variety of other syrups, liquors, and juices, which are all referred to as xe2x80x9csyrupsxe2x80x9d in the context of this invention. In some embodiments of the process, more than about 50% by weight, preferably more than about 75%, more preferably more than about 90% by weight of the invert sugars in the feed syrup pass through the nanofiltration membrane and into the nanofiltration permeate.
It is preferred that the nanofiltration membrane have a molecular weight cutoff of about 150-300 daltons. It is also preferred that, prior to nanofiltration, the feed syrup be pre-filtered through a microfiltration or ultrafiltration membrane. This will produce a microfiltration or ultrafiltration retentate and a microfiltration or ultrafiltration permeate. This permeate is subsequently filtered through the nanofiltration membrane. The microfiltration or ultrafiltration retentate comprises at least one impurity that was present in the feed syrup and is selected from the group consisting of colloids, polysaccharides, and color-forming materials. In especially preferred embodiments of the process, more than about 50% by weight of the colloids, polysaccharides, and color-forming materials in the feed syrup pass into the microfiltration or ultrafiltration retentate.
Optionally, the process can also include the step of diafiltration of the microfiltration or ultrafiltration retentate. This will produce a diafiltration retentate and a diafiltration permeate, and the former will have a reduced sucrose content compared to the microfiltration or ultrafiltration retentate. The diafiltration permeate can be combined with the microfiltration or ultrafiltration permeate prior to nanofiltration.
In the present invention, the crystallization of the nanofiltered material of course produces crystalline sucrose, but also produces a molasses byproduct, which can be recycled into the feed syrup, or can be used for other purposes such as animal feed or fermentation syrup. If this molasses byproduct stream is recycled to the feed syrup, it is usually preferable to withdraw a bleed stream from the recycled byproduct in an amount sufficient to prevent buildup of impurities in the process to an extent that would inhibit crystallization of sucrose.
One specific embodiment of the present invention is a process for obtaining sucrose from molasses. This process includes the steps of:
(a) filtration of molasses that comprises sucrose and no less than about 5% invert sugars (on a dry solids basis), preferably at least about 10% invert sugars using a microfiltration or ultrafiltration membrane, whereby a first permeate and a first retentate are produced, wherein the first retentate comprises at least one impurity that was present in the molasses and is selected from the group consisting of colloids, polysaccharides, and color-forming materials;
(b) nanofiltration of the first permeate using a nanofiltration membrane having a molecular weight cutoff of about 150-300 daltons, whereby a second permeate and a second retentate are produced, wherein the second permeate comprises more than about 75% by weight of the invert sugars that were in the molasses, and wherein the second retentate has a concentration of sucrose that on a dry solids basis is higher than the concentration of sucrose in the molasses; and
(c) crystallization of sucrose from the second retentate.
The present invention provides a relatively simple and low-cost process for enhancing sucrose recovery. The present invention is especially useful for recovery of additional sucrose from molasses, thereby allowing the overall product mix of a sugar manufacturing facility to be sold for a higher aggregate price.
The present invention provides a method of increasing the sucrose content of syrup, and lowering its invert and ash content sufficiently for the sucrose to be recovered by crystallization. For example, a typical molasses from a sugar cane refinery contains 50% sucrose, 23% invert, 17% ash, and 10% other organic components. It is possible to use the process described here to reduce the invert level from 23% to 2%, and reduce the ash level from 17% to 7%, giving a purified syrup with a sucrose content of 75%. The upgraded syrup from this process can be fed to crystallization equipment to recover more sucrose. There will be a small loss of sucrose into the permeate from the nanofiltration membrane, which is the by-product of the nanofiltration process. This permeate material often will have 20% or less sucrose, which gives an 80% yield of sucrose from the syrup. This nanofiltration byproduct would comprise 45% invert, and with a total sugars content of 65% could be sold as a fermentation syrup.